Chert of Interior Alaska

Unit symbol: Oc
Age range Ordovician (485.4 to 443.4 Ma)
Lithology: Sedimentary
Group name: Chert of Interior Alaska
Dark-gray and black banded chert, siliceous slate, argillite, and less abundant greenstone, and fine-grained, thin-bedded, impure limestone and dolostone. Beds separated by carbonaceous partings. Unit interpreted to be a deep-water sequence composed of turbiditic and hemipelagic deposits (Patton and others, 2009). Unit locally contains conodonts and graptolites of Early and Middle Ordovician age in the Medfra quadrangle (Patton and others, 2009) and in adjoining parts of the Mount McKinley quadrangle and has graptolites in the Livengood quadrangle occurrences (Weber and others, 1992). Includes the Livengood Dome Chert in the Livengood, Circle, and Tanana quadrangles (Foster and others, 1983; Weber and others, 1992; J.N. Dover, written commun., 1997), the Nilkoka Group in Fairbanks quadrangle (Péwé and others, 1966), and chert and slate in the Mount McKinley and Kantishna River quadrangles (unit Doc of Bela Csejtey, Jr., written commun., 1993; Chapman and others, 1975). Locally, also included is a distinctive marker bed of lime mudstone within the chert unit, locally characterized by coated ooids in the Tanana and Circle quadrangles (J.N. Dover, written commun., 1997). Thickness varies from 10 to several tens of meters, probably in part due to structural thickening and repetition. Unit exposed in two areas and assigned to three different terranes. The northern exposures of this unit are in the Circle, Livengood, and Tanana quadrangles between thrust splays associated with the Tintina Fault System and are typically considered part of the Livengood terrane (Dover, 1994). The southern exposures are in the Medfra, Ruby, Mount McKinley, and Kantishna quadrangles and are considered part of the Minchumina and Farewell terranes (Patton and others, 2009). Unit may correlate with part of the Road River Formation of eastern Alaska and Yukon. Unit is exposed near the Amy Creek unit of Weber and others (1992), included here in unit DPxkb

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol Oc
Unit name Chert and argillite
Description Banded chert, argillite, fine-grained thin-bedded limestone, and silty dolomite. Beds separated by carbonaceous partings. Unit interpreted to be a deep-water sequence composed of turbiditic and hemipelagic deposits. Eastern part of Medfra quadrangle and southeastern corner of Ruby quadrangle
Lithology Metamorphic

Correlated geologic units

Label Oc
Description Chert and phyllite, Telida sub-terrane of Minchumina terrane
Geologic age Ordovician
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Phyllite < Metaclastic < Metasedimentary < Metamorphic Major
Chert < Chemical < Sedimentary Bed Major